E-SYMMETRY PHONON POLARITONS IN PBTIO3

We have measured the dispersion of the two lowest E-symmetry phonon po lariton branches in PbTiO3 for wave-vector values up to 3000 cm(-1). T he time domain transient grating experimental scheme used is well suit ed to state that at room temperature no pure relaxation motion, specif ically from an order-disorder crystal, can be evidenced. Nevertheless, the polariton dispersion behavior can only be explained by a model wh ich takes into account a weak relaxational mode with an ill-defined or igin. This behavior, deduced from a fit to the static dielectric const ant, is characteristic of the mixed mechanism for ferroelectricity in PbTiO3, as in a lot of other perovskites.